A switch-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a dc input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50%). Additionally, as voltage or current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on a load microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain an output characteristic such as an output voltage at a desired value.
In an exemplary application, the power converters have the capability to convert an unregulated input voltage, such as 12 volts, supplied by an input voltage source to a lower, regulated, output voltage, such as 2.5 volts, to power a load. To provide the voltage conversion and regulation functions, the power converters include active power switches such as metal-oxide semiconductor field-effect transistors (“MOSFETs”) that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor or the primary winding of a transformer to the voltage source at a switching frequency that may be on the order of 500 kHz or higher.
A conventional way to regulate an output characteristic of a switch-mode power converter, such as output voltage, is to sense a current in an inductive circuit element such as an output inductor in a forward converter topology or a transformer primary winding in a forward or flyback converter topology, and compare the sensed current with a threshold current level to control a duty cycle of the power converter. The threshold current level is generally set by an error amplifier coupled to a circuit node such as an output terminal of the power converter to regulate the output characteristic. The mechanism to control duty cycle is a signal to turn a power switch “on” or “off.”
A feedback circuit structure wherein a duty cycle of the power converter is controlled by sensing a current in an inductive circuit element and comparing the sensed current to a threshold that is controlled by an error amplifier is generally referred to as current-mode control. An alternative feedback circuit structure wherein a duty cycle of the power converter is controlled by comparing a triangular waveform generated by an oscillator to a threshold voltage level controlled by an error amplifier is generally referred to as voltage-mode control.
In current-mode control, two feedback loops can usually be identified. In one loop, referred to as the inner current feedback loop, the sensed current is compared with a threshold current level. A second loop, referred to as the outer voltage feedback loop, provides the threshold current level with an error amplifier that senses an output characteristic of the power converter, such as an output voltage. The inner current feedback loop generally becomes unstable in a continuous current mode (“CCM”) of operation when duty cycle increases beyond 50%, regardless of the stability of the outer voltage feedback loop. CCM refers to uninterrupted current flow in an inductor such as an output inductor over a switching period of the power converter. The inner current feedback loop does not become unstable in a discontinuous current mode (“DCM”) of operation. Many power converter designs would suffer serious limitations if duty cycle greater than 50% was not allowed.
The stability of the inner current feedback loop is dependent on loading and the slope of the sensed current versus time. By injecting a small amount of slope compensation into the inner current feedback loop, stability of this loop results for all values of duty cycle. A process to adjust the slope of the sensed current is generally referred to as slope compensation. In one method to provide slope compensation, as described in U.S. Pat. No. 4,672,518, a fixed-slope ramp signal is generated by an oscillator and is summed with a sensed current waveform to produce a slope-compensated current signal. However, under high-current loading of the power converter, the slope compensation signal is disabled, which does not ensure system stability in a continuous conduction mode of operation at high output current levels. While this technique recognizes that a compensating ramp signal may be optimized for one mode of operation, fixed slope compensation is generally suitable only for certain but not all applications.
In another method to provide slope compensation, as described in U.S. Pat. No. 4,837,495, a variable slope ramp signal is generated dependent on the input voltage and the output voltage that is summed with a sensed current waveform for slope compensation. The effect of slope compensation also depends on the value of the circuit inductor value, which is not compensated by this method. Inaccurate slope compensation can produce an overcompensated or undercompensated transient response of the power converter, which is frequently a specified power converter characteristic. In addition, an extra physical pin must generally be provided for the input voltage sensing function.
In a third method to provide slope compensation, as described in U.S. Pat. No. 5,903,452, a fixed slope compensation method is described. In this method, slope compensation is again dependent on a value of inductance and on the input voltage, the effects of which are not compensated by the method, producing thereby inaccurate compensation under certain operating conditions.
Thus, there is a need for a process and related method to provide slope compensation for a sensed current in a switch-mode power converter that avoids the disadvantages of conventional approaches.